The present invention is directed to a vapor vent valve especially adapted for use on gasoline tank trucks to accommodate the flow of fuel vapor to or from the headspace of the tank during a fuel loading or unloading operation. The present invention is directed to improvements to the vapor vent valve which is the subject of the above-identified parent application.
When gasoline is transferred from a storage tank at the supply terminal into the delivery truck tank, and again when the gasoline is transferred from the delivery truck tank to the underground storage tank at a service station, fuel vapor must be expelled from the headspace of the tank receiving the fuel. Present-day environmental standards prohibit the discharge of this expelled fuel vapor into the atmosphere, and the standard present-day practice is to connect the headspaces of the delivering tank and the receiving tank to each other so that fuel vapor expelled from the receiving tank by the incoming fuel is conducted into the headspace of the delivering tank to maintain a constant headspace pressure in the delivering tank. At the conclusion of the fuel transfer operation, the headspace of each of the two tanks is sealed.
A vent valve for a delivery truck tank will most logically take the form of a normally closed valve, held closed by a spring. Compressed air for operating the truck brakes is readily available, and the utilization of a pneumatic motor for opening the vent valve is quite conventional. The pressure differential across the valve, when closed, is normally quite small, hence the biasing spring need only be strong enough to hold the valve closed in the face of bumps and shock loading encountered during movement of the truck. In a valve of this type, the weakest point of the system is the piston seal of the pneumatic motor. Leakage past the valve head--valve seat seal, while undesirable, is not a large problem in that the pressure differential across the seal is so small that the amount of vapor leakage is minimal. Leakage or failure of the piston seal, on the other hand, can prevent actuation of the valve. In that the motor is actuated only infrequently and a high degree of precision is not required, the tendency is to fabricate the motor as cheaply as possible.
These vent valves are mounted in an opening in the top of the tank, the valve housing being effectively a short, reasonably large diameter pipe section extending vertically through the top wall of the tank with its lower end opening into the interior of the tank and its upper end opening at the exterior of the tank. The valve head can consist simply of a circular plate located at the lower end of the valve housing and biased upwardly by springs to seat against and close the lower end of the passage through the valve housing. The pneumatic motor is almost invariably mounted within the passage through the housing with its piston rod extending downwardly and connected to the vent valve head. The open upper end of the valve housing is sealed by a detachable hood coupled to a vapor conducting conduit through which fuel vapor flows to or from the upper end of the valve passage. With this arrangement, access to the valve interior is through the open upper end of the valve housing which in turn is accessible on removal of the hood referred to above.
With many of the prior art valves, it is not possible to service the valve, as by replacing a piston seal, for example, without removing the entire valve assembly from the tank truck. The present invention is directed to an arrangement in which a disposable valve actuating motor may be readily removed as a unit from the valve while the valve housing remains in place on the truck tank.